Method for improving erasability or erasable marking compositions

ABSTRACT

A method is disclosed for improving the erasability of erasable marking compositions, while maintaining the opacity of the coating and without producing burnishing, by incorporating into the compositions voided latex particles having a shell polymerized from either: (1) at least about 25% by weight, based on the weight of the shell, of a crosslinking comonomer; (2) at least about 5% by weight, based on the weight of the shell, of a crosslinking comonomer, when at least about 50% by weight of methyl methacrylate is used as a comonomer in the shell. An improved liquid marking composition is also disclosed.

FIELD OF THE INVENTION

This invention relates to a method for improving the erasability oferasable marking compositions by incorporating into the compositionsvoided latex particles having a crosslinked shell.

BACKGROUND OF THE INVENTION

Chalk used to make markings on blackboards or chalkboards is typicallymade from calcium carbonate formed into a hard solid stick or cake. Whenthe chalk is used, calcium carbonate is transferred to the blackboardsurface. A large amount of calcium carbonate dust is also generated whenthe markings are made and when the blackboard erasers used to wipe awaythe markings are cleaned by slapping them together. This dust is morethan just a housekeeping nuisance, it poses health concerns, especiallyin a classroom setting, because the inhalation of the dust may cause,inter alia, respiratory problems. In addition, the dust is harmful toelectronic equipment, such as computers. Therefore, it is desirable toproduce a marking medium useful on a standard blackboard or chalkboardwhich does not pose health and safety concerns.

Efforts have been made to produce marking media, such as, for example,liquid marking compositions, which do not generate dust or have otherdeleterious health effects. There has only been limited success indeveloping liquid marking compositions because, in the effort to producea healthier marking medium, performance properties of the compositionshave been compromised.

One problem is that the liquid marking compositions do not have the samewhiteness or opacity on conventional blackboards as conventional chalk.Opacifiers have been added to solve this problem. As a result of theaddition of the opacifying agents, however, many of the compositionslose their "erasability," i.e., they leave a hazy, whitish film on thedark blackboard or chalkboard after the board has been wiped with aconventional blackboard eraser or additionally wiped with water. Theformation of this undesirable film is referred to as "ghosting."

Another problem with liquid marking compositions has been burnishing ofthe blackboard, i.e., a glazing, not associated with any physicalabrasion, of the blackboard surface caused by a component of the liquidmarking composition, such as soft, low melting point polymers and waxymaterials.

SUMMARY OF THE INVENTION

I have discovered a method for improving the erasability of erasablemarking compositions, while maintaining the opacity of the coating andwithout producing burnishing, by incorporating into the compositionsvoided latex particles having a shell polymerized from either:

(1) at least about 25% by weight, based on the weight of the shell, of acrosslinking comonomer; or

(2) at least about 5% by weight, based on the weight of the shell, of acrosslinking comonomer, when at least about 50% by weight of methylmethacrylate is used as a comonomer in the shell.

DETAILED DESCRIPTION OF THE INVENTION

The composition and method of this invention involve incorporating intoconventional liquid marking compositions voided latex particles havingcrosslinked shells. The voided latex particles containing a core and ashell may be prepared by conventional polymerization techniques such as,sequential emulsion polymerization, including those processes disclosedin U.S. Pat. Nos. 4,427,836; 4,594,363; 4,677,003 4,920,160; 4,970,241,whose disclosures are incorporated herein by reference, and EuropeanPatent Application 0,267,726.

The methods disclosed in the aforementioned patents include a step ofswelling the multistaged polymer particles with water or solvent in thewet state. Thus, the polymer particles do not become voided until thewater or solvent inside the particles evaporates. Generally, this doesnot occur until the coating systems to which the polymer particles areadded are allowed to dry. Because the polymer particles useful in thecomposition and method of this invention have a shell formed from acrosslinking monomer, they are resistant to swelling. Thus, it ispreferable to polymerize the shell polymer after most of the swelling ofthe core has occurred, i.e., after the addition of base for an acid-richcore; after the addition of acid for an amine-rich core; and after theaddition of solvent for a solvent-swellable core.

The voided latex particles useful in composition and method of thisinvention are formed from a multistaged polymer particle having a corepolymer and a shell polymer, wherein the shell polymer is polymerizedfrom either:

(1) at least about 25% by weight, based on the weight of the shell, of acrosslinking comonomer; or

(2) at least about 5% by weight, based on the weight of the shell, of acrosslinking comonomer, when at least about 50% by weight of methylmethacrylate is used as a comonomer in the shell.

The balance of the monomers used in the aqueous emulsion polymerizationof the shell polymer may be at least 1% by weight one or moremonoethylenically unsaturated monomers containing at least onecarboxylic acid group and one or more non-ionic ethylenicallyunsaturated monomer.

"Crosslinking comonomer" as used herein refers to a polyfunctionalmonomer or mixture of monomers which crosslinks a polymer compositionduring the initial formation thereof. Subsequent drying or other curingtechniques are not required. Comonomers of this type are well-known andinclude monomers wherein the functionality is of substantiallyequivalent reactivity so that uniform crosslinking occurs. Typically,such comonomers contain at least two addition polymerizable vinylidenegroups and are α,β-ethylenically unsaturated monocarboxylic acid estersof polyhydric alcohols containing 2-6 ester groups. Suitablecrosslinking comonomers include alkylene glycol diacrylates anddimethacrylates, such as for example, ethylene glycol diacrylate,ethylene glycol dimethacrylate, 1,3-butylene glycol diacrylate,1,4-butylene glycol diacrylate propylene glycol diacrylate andtriethylene glycol dimethylacrylate; 1,3-glycerol dimethacrylate;1,1,1-trimethylolpropane dimethacrylate; 1,1,1-trimethylolethanediacrylate; pentaerythritol trimethacrylate; 1,2,6-hexane triacrylate;sorbitol pentamethacrylate; methylene bis-acrylamide, methylenebis-methacrylamide, divinylbenzene, vinyl methacrylate, vinyl crotonate,vinyl acrylate, vinylacetylene, trivinylbenzene, triallyl cyanurate,divinylacetylene, divinylethane, divinyl sulfide, divinyl ether, divinylsulfone, diallyl cyanamide, ethylene glycol divinyl ether, diallylphthalate, divinyldimethylsilane, glycerol trivinyl ether, divinyladipate; dicyclopentenyl (meth)acrylates; dicyclopentenyloxy(meth)acrylates; unsaturated esters of glycol monodicyclopentenylethers; allyl esters of α,β-unsaturated mono- and dicarboxylic acidshaving terminal ethylenic unsaturation including allyl methacrylate,allyl acrylate, diallyl maleate, diallyl fumarate, diallyl itaconate andthe like. Allyl methacrylate is preferred. The term (meth)acrylate, asused herein, refers to both acrylate and methacrylate.

Suitable monoethylenically unsaturated monomers containing at least onecarboxylic acid group include acrylic acid and methacrylic acid,acryloxypropionic acid, (meth)acryloxypropionic acid, itaconic acid,aconitic acid, maleic acid or anhydride, fumaric acid, crotonic acid,monomethyl maleate, monomethyl fumarate, and monomethyl itaconate.Acrylic acid and methacrylic acid are preferred.

Suitable non-ionic ethylenically unsaturated monomers include styrene,vinyltoluene, ethylene, vinyl acetate, vinyl chloride, vinylidenechloride, acrylonitrile, (meth)acrylamide, (C₁ -C20) alkyl or (C₃ -C₂₀)alkenyl esters of (meth)acrylic acid, such as methyl (meth)acrylate,ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,benzyl (meth)acrylate, lauryl (meth)acrylate, oleyl (meth)acrylate,palmityl (meth)acrylate and stearyl (meth)acrylate. As used herein, theterm "(meth)acrylic" is intended to serve as a generic expressionembracing both acrylic and methacrylic.

The core polymer may be the product of aqueous emulsion polymerizationof one or more monoethylenically unsaturated monomers containing atleast one carboxylic acid group. The core polymer may be obtained by theemulsion homopolymerization of such a monoethylenically unsaturatedmonomer containing at least one carboxylic acid group or bycopolymerization of two or more monoethylenically unsaturated monomerscontaining at least one carboxylic acid group. Preferably, themonoethylenically unsaturated monomer containing at least one carboxylicacid group is copolymerized with one or more non-ionic (that is, havingno ionizable group) ethylenically unsaturated monomers.

The core polymer may also contain from about 0.1% to about 20% byweight, based on the weight of the total monomer weight of the core,preferably about 0.1% to about 3% by weight of polyethylenicallyunsaturated monomer, such as ethylene glycol di(meth)acrylate, allyl(meth)acrylate, 1,3-butane-diol di(meth)acrylate, diethylene glycoldi(meth)acrylate, trimethylolpropane trimethacrylate, or divinylbenzene.Alternatively, the core polymer may contain from about 0.1% to about 60%by weight, based on the weight of the total monomer weight of the core,of butadiene.

The voided latex particles useful in the method of this invention areincorporated into the conventional liquid marking composition at a levelof from about 5% to about 40%, based on the total weight of the liquidmarking composition, and preferably from about 10% to about 25%, basedon the total weight of the liquid marking composition.

The composition and method of this invention are useful for improvingthe erasability of both conventional aqueous-based and solvent-basedliquid marking compositions. Conventional aqueous-based liquid markingcompositions may contain water, volatile watermiscible solvents, such asmethanol, to increase the rate of drying, soluble dyes, fine pigmentparticles, dispersants and thickeners. Conventional solvent-based liquidmarking compositions may contain solvent, dyes, fine pigment particles,dispersants and thickeners.

The following examples illustrate the operation of the invention. Theseexamples are intended merely to illustrate the invention and are notintended nor should they be interpreted as limiting the scope of theinvention since modifications to the process illustrated are consideredto be obvious to one of ordinary skill in the art.

EXAMPLE 1 Preparation of Liquid Marking Compositions

Comparative voided latex particles and voided latex particles useful inthe composition and method of the invention were prepared by the methoddescribed in U.S. Pat. No. 4,427,836. Excess ammonia (1.3 equivalentsbased on the total equivalents of methacrylic acid in the particles) wasadded to the hot (8020 -85° C.) dispersion between the polymerizaton ofshell II and shell III to convert the acid-rich core to the ammoniumsalt causing the core to swell with water. The final particle sizesaveraged 400-550 nm and the solid content of the dispersions ranged from32-43% by weight. All of the dispersions were diluted with water to 20%solids by weight to give the final liquid marking composition.

                                      TABLE 1.1                                   __________________________________________________________________________    Compositions of the Voided Latex Polymers                                     1 part core*//9 parts(20 BMA/78 MMA/2 MAA)(Shell I)//x parts(Shell II)//      y parts(Shell III)*140 nm,5 BA/55 MMA/40 MAA                                  Voided Latex                                                                          Shell II     Shell III                                                Particles                                                                             x  composition                                                                             y  composition                                           __________________________________________________________________________    1C      22.5                                                                             100 Sty   22.5                                                                             100 Sty                                               2C      22.5                                                                             100 Sty   22.5                                                                              60 Sty/40 DVB-55                                     3C      22.5                                                                             100 Sty   22.5                                                                              75 Sty/25 ALMA                                       4C      22.5                                                                             100 Sty   22.5                                                                              85 Sty/15 ALMA                                       5.sup.  20  90 MMA/10 Sty                                                                          20  75 MMA/10 Sty/15 ALMA                                6.sup.  4  100 Sty   9   40 Sty/60 DVB-55                                     __________________________________________________________________________    Notes:                                                                        The suffix "C" denotes a comparative.                                         Abbreviations                                                                 MMA  methyl methacrylate                                                      BMA  butyl methacrylate                                                       BA   butyl acrylate                                                           MAA  methacrylic acid                                                         Sty  styrene                                                                  DVB-55                                                                             Dow 55% grade of divinylbenzene where most of remainder is                    ethylvinylbenzene                                                        ALMA allyl methacrylate                                                   

EXAMPLE 2 Performance Testing

Markers containing the formulations made in accordance with Example 1were tested for erasability, burnishing and opacity by marking on both aslate chalkboard and a composition chalkboard at both one cycle and 50cycles. One cycle is covering a 1-inch-by-2-inch rectangle on thechalkboard surface and erasing the marking using a conventionalchalkboard eraser. Fifty cycles is repeating the process 50 times.

Erasability Testing

The erasability of each formulation was judged by how well it could beremoved, using a conventional chalkboard eraser, in comparison to theerasability of a similar marking made using conventional calciumcarbonate chalk. The results are shown in Table 2.1.

Burnishing Testing

The burnishing of each formulation was judged by how much glazing of thechalkboard surface appeared which was not a result of physical abrasion.The results are shown in Table 2.1.

Opacity Testing

The opacity of each formulation was judged by how well the markingdeveloped solid whiteness when dry. The results are shown in Table 2.1.

                                      TABLE 2.1                                   __________________________________________________________________________    Performance Results                                                           Formulation containing                                                        Voided Latex Polymer                                                                       Erasability                                                                             Burnishing                                                                           Opacity                                         __________________________________________________________________________    1C           ghosting - fail                                                                         yes - fail                                                                           good (white)                                    2C           ghosting - fail                                                                         yes - fail                                                                           fair (white)                                    3C           ghosting - fail                                                                         yes - fail                                                                           fair (white)                                    4C           ghosting - fail                                                                         yes - fail                                                                           good (white)                                    5.sup.       good (no ghosting) -                                                                    no - pass                                                                            good (blue-white)                                            pass                                                             6.sup.       good (no ghosting) -                                                                    no - pass                                                                            good (white)                                                 pass                                                             __________________________________________________________________________

The results indicate that formulations containing voided latex particleshaving a shell polymerized from either:

(1) at least about 25% by weight, based on the weight of the shell, of acrosslinking comonomer; or

(2) at least about 5% by weight, based on the weight of the shell, of acrosslinking comonomer, when at least about 50% by weight of methylmethacrylate is used as a comonomer in the shell exhibit gooderasability (no ghosting) while maintaining the opacity of the coatingand without producing burnishing.

I claim:
 1. A method of improving the erasability of an erasable markingcomposition, comprising incorporating into said composition voided latexparticles formed from a multistaged polymer particle having a corepolymer polymerized from ethylenically unsaturated monomers and having ashell polymerized from at least about 25% by weight, based on the weightof the shell, of an ethylenically unsaturated crosslinking comonomer. 2.A method of improving the erasability of an erasable markingcomposition, comprising incorporating into said composition voided latexparticles formed form a multistaged polymer particle having a corepolymer polymerized from ethylenically unsaturated monomers and a shellpolymerized from at least about 50% by weight of methyl methacrylate andfrom at least about 5% by weight, based on the weight of the shell of anethylenically unsaturated crosslinking comonomer.
 3. The method ofclaims 1 or 2 wherein said crosslinking comonomer is a comonomerselected from the group consisting of alkylene glycol diacrylates,alkylene glycol dimethacrylates, 1,3-glycerol dimethacrylate,1,1,1-trimethylolpropane dimethacrylate, 1,1,1-trimethylol ethanediacrylate, pentaerythritol trimethacrylate, 1,2,6-hexane triacrylate,sorbitol pentamethacrylate, methylene bis-acrylamide, methylenebis-methacrylamide, divinylbenzene, vinyl methacrylate, vinyl crotonate,vinyl acrylate, vinyl acetylene, trivinyl-benzene, triallyl cyanurate,divinylacetylene, divinylethane, divinyl sulfide, divinyl ether, divinylsulfone, diallyl cyanamide, ethylene glycol divinyl ether, diallylphthalate, divinyldimethylsilane, glycerol trivinyl ether, divinyladipate, dicyclopentenyl (meth)acrylates; dicyclopentenyloxy(meth)acrylates, unsaturated esters of glycol monodicyclopentenylethers, allyl esters of α,β-unsaturated monocarboxylic acids havingterminal ethylenic unsaturation and allyl esters of α,β-unsaturateddicarboxylic acids having terminal ethylenic unsaturation.
 4. The methodof claims 1 or 2 wherein said crosslinking comonomer is a comonomerselected from the group consisting of an allyl ester of α,β-unsaturatedmonocarboxylic acid having terminal ethylenic unsaturation andα,β-unsaturated dicarboxylic acid having terminal ethylenicunsaturation.
 5. The method of claims 1 or 2 wherein said crosslinkingcomonomer is allyl methacrylate.
 6. The method of claims 1 or 2 whereinsaid voided latex particles have a core formed from at least oneacid-functional comonomer and wherein said shell is polymerized afterthe addition of a base to said core.
 7. The method of claims 1 or 2wherein said voided latex particles have a core formed from at least oneamine-functional comonomer and wherein said shell is polymerized afterthe addition of an acid to said core.
 8. The method of claims 1 or 2wherein said voided latex particles have a solvent-swellable core andwherein said shell is polymerized after the addition of a solvent tosaid core.